Process for reinforcing glass objects

ABSTRACT

The invention relates to a process for reinforcing a glass object by precompressing the surface of the object, acid etching said surface to a depth equal to or less than the compressed depth of the glass, and finally protecting the etched surface.

The invention relates to procedures for increasing the mechanicalstrength of glass objects, particularly flat or curved glass plates. Itin particular applies to aircraft glazings or more generally to allcases where a high mechanical strength must be ensured.

It is known that the mechanical strength of a glass object can beincreased by a tempering operation consisting of placing the surface ofthe object under compression beforehand either thermally or chemicallyby an ion exchange process, which leads to the surface replacement ofpart of the sodium ions by larger ions such as e.g. potassium ions,which thus place the surface of the object under compression.

The final mechanical strength obtained is obviously dependent on thetype of tempering treatment performed, but also on the quality of thesurface prior to the treatment of the object. In the case of a glassplate, the most important surface defects are essentially encounteredclose to edges which have undergone a cutting treatment. It is certainlypossible to minimize this problem by a high-quality polishing of theedges, e.g. using a so-called polished, full round edge method, but thistreatment is relatively complicated and there is always a risk of partof the edges being incompletely polished. Moreover, the defects are notsolely located on the edges, even when the glass is obtained by aprocess not normally requiring any making good by machining and inparticular by the presently most widely used process, namely the floatprocess, or other standard processes such as the fusion draw process,the Fourcault process or rolling processes with or without marks.

Thus, on measuring the mechanical strength of a large number of temperedplates, there is found to be relatively wide dispersion of the effectivetempering levels and that even if the mean value is significantly higherthan the mean value of the mechanical strength of an untempered plate,certain plates still have mechanical strength values substantiallyidentical to the initial mechanical strength. Moreover, the breakingstress levels can sometimes, particularly in the case of very largeplates, such as e.g. those intended for aircraft windscreens, proveinadequate for the envisaged application.

It is in particular known from FR-A-2 138 710 or its equivalent U.S.Pat. No. 3,843,472 to reinforce a glass article by smoothing by anabrasion process part of the surface and/or edge, which then undergoes,before and/or after the chemical tempering of the glass, to a so-calledgrinding treatment by means of an acid agent such as an aqueous solutionof hydrofluoric acid and optionally sulphuric acid. The shock resistanceobtained by the combination of the abrasion treatment and the grindingtreatment is superior to that obtained when only one of the treatmentsis performed prior to the thermal tempering and well above that obtainedwhen the glass only undergoes a grinding using hydrofluoric acid.

Thus, hydrofluoric acid grinding alone does not make it possible toeliminate major surface defects because the etching is not selective.The concave parts of the defects are reached in the same way and theconvex parts, so that essentially there is only a reproduction bytranslation of the geometry of the glass surface and consequently thatof the defects. However, a slight, but not usually significant gain isobtained, because the treatment makes it possible to slightly widen thedefects and therefore the mechanical strength is slightly increased.

In order to be effective, the known process of the aforementioned patentrequires a careful abrasion with a treatment of the edges by asuccession of flexible abrasive belts, which are oriented differentlyand which differ by the nature and size of the grains used. Such atreatment takes a long time and must be carried out by particularlyskilled workers, without it being possible to check the quality of thetreatment by any tests which are not destructive and this can obviouslynot be performed in a systematic manner.

Moreover, although it is true that most defects are produced by cuttingoperations and are therefore located on the edges, the main faces of theglass objects are still not perfect, even in the case of a processrequiring no machining of the faces, such as the float process. However,it is possible to attempt to obviate this by polishing, but once againit is a long and costly supplementary treatment, where the slightesterror can give rise to defects worse than those which it is aimed toeliminate. Any supplementary manipulation of the glass can lead to theformation of new defects. It is therefore agreed that articles formingthe object of such reinforcing treatments are those for which thequality requirement is very high from the very first production stage,so that the number of defects is lower than for a more ordinary product.

Under these conditions, certain of the objects produced have poormechanical strengths and even if it is statistically rare, it isnevertheless true that these few cases constitute a major problem whenit is a question of producing an aircraft wind-screen or ahigh-security, armoured glazing.

The present invention is directed at an industrial process forincreasing the mechanical strength of glass objects of random sizes,over the entire surface thereof, no matter whether or not they haveundergone machining of the edges, whilst making it possible to obtainparticularly high breaking stress levels and more especially a limiteddispersion of the mechanical strength values, particularly with respectto the low points.

According to the invention this object is achieved by a reinforcingprocess consisting of the prior compressing of the surface of a glassobject, an acid etching of said surface over a depth equal to or belowthe compressed glass depth and a protection of the surface treated byacid etching.

Thus, the process according to the invention consists of the combinationof a tempering pretreatment of the object surface, either thermally orchemically, followed by an acid etching of the surface, which issubsequently protected either extrinsically by a protective layer, or,in preferred manner, intrinsically by a further tempering.

The prior compression of the surface of the object also ensures thecompression prestress of defects limiting as a result of this thediffusion to the bottom of the crack of acid and therefore the relativeelongation of the defect with respect to the etched surface. Finally,this leads to a reduction in the size of the defect to that ensuring avery significant breaking stress increase, but which is still a functionof the tested surface.

The prior compression pretreatment is carried out in such a way as toensure a compressed layer having an adequate prestress amplitude anddepth with respect to what is considered to be the largest probabledefect on the considered surface, said defect size being proportional tothe surface and, for the edges, is dependent on the type of working used(abrasive belt, diamond grinding wheel, etc.). The depth removed by theacid will be a direct function of the size of the largest estimateddefect on the surface and in the limiting case could reach the depth ofthe compressed layer.

The pretreatment has both a purely mechanical function, namely ensuringa compression prestress, and a chemical function, because it is able toensure a homogenization of the etching front coupled with the mechanicalprestress. The inventors have found that the higher the etchingkinetics, i.e. the ablation speed for a given surface, the greater mustbe the prestress. Correlatively, for weak acid concentrations, aprestress by thermal tempering and therefore of a smaller amplitude thana prestress by the chemical procedure, but affecting a larger depth,makes it possible to obtain results similar to those obtained for aglass which has previously been chemically tempered. However thermaltempering leads to the disadvantage for small surfaces of only having anaverage geometrical quality, accompanied by a generation of defectsduring the heating and transportation of the glass before and duringtempering. Therefore, said pre-treatment is preferably performed bychemical tempering, because it then leads to a significant increase inthe number of alkali metal ions on the surface, which tends to limit theetching kinetics by the acid and thus homogenizes the etching front.

Following acid etching, a surface of the virgin surface type isobtained, identical to a freshly formed surface, having very few defectsor at least only having defects which are sufficiently small to ensurethat the mechanical strength of the surface is very high. However, thisproperty only remains true if no new defects are generated of asufficiently large size to reduce the mechanical strength once again.However, it is known that a virgin surface is in fact very sensitive andundergoes a very rapid deterioration as soon as it is brought into thepresence of any material liable to form a crack. Thus, a simple handlingoperation giving rise to contacts is sufficient to damage the surfaceobtained by such a treatment, so that it should be protected.

In a first variant of the invention, said protection is providedextrinsically by the deposition, as from the end of the acid etching, ofa mechanical barrier (resistance to scratching/identation) and/orchemical barrier (resistance to corrosion by water, acids, etc.)deposited under clean conditions ensuring the absence of hard particlesbetween the glass and the deposit, which could give rise to indentationsduring use. These protective deposits can be in the form of polymers,mineral and/or organic layers, of a metallic nature, etc. Preferably,these layers are directly formed on the surface of the object, but itwould also be possible to use films, particularly polyurethane films,provided that a stoving took place (the film must e.g. be heated toabout 100° C.) under particularly strict cleanness conditions.

Preferably, the metallic oxide layers are constituted by Ta₂ O₅ -typelayers, which are known to have a good mechanical strength and aspolymer films are used polyurethane films, particularly a crosslinkedpolyurethane, e.g. deposited by dip coating or, in preferred manner, byspraying. It is also possible to use polyvinyl butyral films, but theyare not generally preferred due to their hydrophilic character.

Polymer films, and in the same way metallic or metal oxide films,deposited for the protection of the glass surface after acid etching canalso fulfil an advantageous function in the final glazing. Thus,polyurethane films can have an action against scratching, can be able tomaintain in place glass splinters in the case of the glazing beingshattered, can absorb stresses, or can serve as an interface between theglass and an assembly polymer film such as a polyvinyl butyral film,particularly with a view to reducing the cold spalling effect due to theexpansion difference at low temperature between the glass and thepolyvinyl butyral. Moreover, if said polyurethane film is deposited onthe six faces of the glass sheets, it permits a true encapsulationthereof, which in itself constitutes a very good protection.

In a second and more particularly preferred variant of the invention,said protection is obtained intrinsically by applying a newreinforcement to the glass. This reinforcement is preferably obtained bya chemical tempering, ensuring that the latter is performed just afterthe chemical etching and limiting the heating time prior to immersion inthe chemical reinforcement bath, maintaining at a high temperaturecausing a reduction of the intrinsic strength of the etched surface byactivating corrosion kinetics at the bottom of the crack which werepreviously reduced. This leads to a product having all the handlingadvantages of conventional, chemically reinforced products, as well astheir resistance to damage, the latter being a direct function of theexchanged depth and the installed mechanical prestress. It is alsopossible to protect the glass plate by a thermal tempering, but if it iswished to retain the integrity of the two surfaces, it is necessary toproscribe the conveying of the glass on rollers and ensure the heatingand tempering either by an air cushion system, or between grippers, orany other process limiting contact.

It is sometimes simpler to protect the glass immediately following theacid etching in an extrinsic rather than an intrinsic manner, but theprotection obtained is less, except when using a thin film polymer,which limits the applications of the treated object, because the polymerfilm is not always able to withstand a use on the external face.

An interesting use of the extrinsic protection can be in the form of aprovisional protection, having a limited, but adequate life in order tobe able to install an intrinsic protection of the chemical reinforcementtype.

It should be noted that on directly carrying out an acid etching(followed by tempering) without any reinforcing pretreatment by thermalor chemical tempering, it is possible to obtain, if the acidconcentration is well chosen, a very high, final strength of theproduct. However, it should be noted that the dispersion of the results(lowest, respectively highest level of a treated batch of samples) isincreased when the etching speed increases, whereas the mean value ofthe breaking stress decreases. The absence of a reinforcing pretreatmentby thermal or chemical tempering does not make it possible to obtain ona given glass batch, a minimum mechanical strength exceeding that of theinitial glass and therefore uniformity of the chemical etching betweenthe surface and large defects. However, the mechanical strength valuesare as high as with a reinforcing pretreatment. Only the dispersion isgreater and does not make it possible to ensure a minimum, mechanicalstrength value greater than that of the basic glass.

Certain applications require a minimum strength value well above that ofthe initial glass, by reducing the lower limit of the dispersion, whichcan only be ensured by the chemical or thermalreinforcement/additives+hydrofluoric acid etching/chemical or thermalreinforcement or protection by mechanical/chemical barrier layerdeposition.

It should also be noted that the addition of different additives, suchas sulphuric acid, makes it possible to obtain the initial opticalquality of the glass and improve it if appropriate. It is moreparticularly advantageous to use an etching solution containingsurfactants, which render the etching uniform.

Other advantageous features and details of the invention can be gatheredfrom the comparative tests summarized hereinafter, all performed on thebasis of float glass plates, whose chemical composition is in accordancewith the following formulation, expressed in weight percentages:

    ______________________________________                                               SiO.sub.2                                                                              71.7                                                                 Al.sub.2 O.sub.3                                                                       0.6                                                                  CaO      9.5                                                                  MgO      4                                                                    Na.sub.2 O                                                                             13.6                                                                 So.sub.3 0.2                                                                  Miscellaneous                                                                          0.4                                                           ______________________________________                                    

Moreover, the sum of the components individually in weight percentagesis below 0.1%.

300×150×6 mm panels were cut with a diamond reel and the edges underwenta mechanical abrasion either simply of the "dropped edges" type, thetreatment normally applied to any cut glazing so as to prevent it frominjuring the worker and which consists of giving a bevelled profile withthree canted corners, or of the "polished, full round edge" type, i.e.the edge of the glass is completely rounded in order to be shaped like acircumferential portion, the treatment being completed by polishing withalumina.

Chemical tempering (TC) was carried out by placing the panels in apotassium nitrate bath at 460° C. and for 48 hours. The exchanged depthis approximately 40 to 60 microns.

The acid etching is performed with the aid of an aqueous 2.4% by volumehydrofluoric acid solution at a temperature of 24° C. For etching adepth of approximately 80 microns, the etching is carried out for 4hours.

The results obtained are given in the following table:

    ______________________________________                                        Preparation of edges                                                                          Treatment   Strength                                          ______________________________________                                        None (before cutting)                                                                         --            100 MPa                                         Polished, full round edge                                                                     TC          100-350 MPa                                       Dropped edges   TC            300 MPa                                         Dropped edges   HF           60-800 MPa                                       Dropped edges   HF + TC     120-130 MPa                                       Dropped edges   TC + HF     300-800 MPa                                       Dropped edges   TC + HF + TC                                                                              480-650 MPa                                       Polished, full round edge                                                                     TC + HF + TC                                                                              450-350 MPa                                       ______________________________________                                    

In the two latter cases, it should be noted that the mean strengthmeasured is approximately 550 MPa, i.e. exceptionally high. It shouldalso be noted that the polishing of the edges (polished, full roundedge) does not lead to an increased strength and it is thereforepointless to carry out such a treatment, which is much more expensivethan the mere dropping or reduction of the edges. This represents animportant difference with the results obtained in the prior art bythermal tempering only, the dropped edge method being virtually unusablewhen there is a large number of breakages (even if certain glazingsobtained have a strength of approximately 300 MPa, most of the glazingsmust be considered as having a strength of 0 MPa due to the breakage).In particular, it is possible to obtain objects having a strength exceeding 200 MPa for a thickness of 3 mm, with a thickness under compressionof more than 600 microns. For comparison, a standard car glazing, whichhas undergone a thermal tempering has a strength of approximately 100 to150 MPa, for the same thickness under compression, whereas a chemicaltempering does not generally make it possible to obtain an exchangedepth exceeding 100 microns.

It should be noted that the glass composition referred to hereinbeforecorresponds to an "ordinary" window glass composition of the soda-limeglass type, i.e. not optimized for a chemical tempering and themechanical strengths obtained by the process according to the inventionare significantly higher when use is made of bases produced because oftheir suitability for higher tempering levels.

In this connection reference is e.g. made to a glass, whose compositionis defined by the following weight percentages:

    ______________________________________                                                SiO.sub.2    65 to 76                                                         B.sub.2 O.sub.3                                                                            0 to 4                                                           Al.sub.2 O.sub.3                                                                           1.5 to 5                                                         MgO          4 to 8                                                           CaO          0 to 4.5                                                         Na.sub.2 O   10 to 18                                                         K.sub.2 O    1 to 7.5                                                 ______________________________________                                    

These elements representing at least 96% of the weight of the glass andalso respecting the weight ratios CaO:CaO+MgO between 0 and 0.45 and K₂O/K₂ O+Na₂ O between 0.05 and 0.35, such compositions make it possibleto obtain high reinforcements for large change depths.

The process according to the invention has numerous applications. It cancertainly be used for treating glass plates, but also for treating otherobjects such as bottles.

If the reinforcement treatments are of the thermal tempering type,particularly high thickness depths under compression are obtained, whichare combined with remarkably high stress levels for this type oftempering.

I claim:
 1. A process for the reinforcement of a glass object,comprising:acid etching of a previously compressed surface of saidobject over a depth equal to or greater than the compressed glass depth,and final protection of the etched surface.
 2. Process according toclaim 1, characterized in that said protection is obtained intrinsicallyby a thermal or chemical tempering treatment.
 3. Process according toclaim 1, characterized in that said protection is obtained extrinsicallyby depositing a layer.
 4. Process according to claim 3, characterized inthat the object subsequently undergoes a chemical tempering treatment.5. The product produced by the process of claim
 4. 6. Process accordingto claim 1, characterized in that the acid etching is performed with anagent containing hydrofluoric acid.
 7. Process according to claim 6,characterized in that said agent also contains surfactants.
 8. Theproduct produced by the process of claim
 1. 9. The process according toany one of claims 2, 3, 4 or 1, wherein said previously compressedsurface has been compressed by thermal tempering.
 10. The processaccording to any one of claims 2, 3, 6, 4 or 1, wherein said previouslycompressed surface has been compressed by chemical tempering.
 11. Aprocess for reinforcing a glass object, comprising:compressing a surfacelayer of a glass object; etching said glass object, at least throughsaid surface layer, thereby exposing a new surface; and protecting saidnew surface intrinsically or extrinsically.
 12. The process of claim 11,wherein said protecting comprises compressing said new surface.
 13. Theprocess of claim 12, wherein said compressing of said new surface ischemical tempering.
 14. The product produced by the process of claim 13,wherein said glass object has a strength exceeding 450 MPa.
 15. Theprocess of claim 11, wherein said compressing of said surface layer ischemical tempering.
 16. The process of claim 11, wherein said protectingof said new surface comprises depositing a layer on said new surface.17. The process of claim 16, further comprising subsequent chemicaltempering.
 18. The product produced by the process of claim
 11. 19. Atempered, etched and protected glass object, whose strength exceeds 200MPa for a thickness of 3 mm.
 20. A chemically tempered, etched andprotected glass object, whose strength exceeds 450 MPa.